The present invention relates to the techniques of chemical vapor infiltration that are used in particular when making parts out of thermostructural composite material. The invention relates more particularly to densifying thin porous substrates by deposing a matrix, i.e. densifying substrates that present thicknesses that are relatively small compared with their main dimensions.
In order to fabricate parts out of composite material, in particular parts made out of thermostructural composite material constituted by a refractory fiber preform (e.g. made of carbon or ceramic fibers) that is densified by a refractory matrix (e.g. of carbon or ceramic), it is common practice to make use of chemical vapor infiltration methods. Examples of such parts are thruster nozzles made of carbon-carbon (C—C) composite, or brake disks, in particular for airplane brakes, likewise made of C-C composite.
Densifying porous substrates by chemical vapor infiltration consists in placing the substrates in a reaction chamber of an infiltration installation by using support tooling, and in admitting a reactive gas into the chamber, which gas contains one or more precursors of the material that is to be deposited within the substrates in order to densify them. Infiltration conditions, in particular the composition and the flow rate of the reactive gas, and also the temperature and the pressure within the chamber, are selected to enable the gas to diffuse within the accessible internal pores of the substrates so as to deposit the desired material therein by decomposing a constituent of the gas or by a reaction between a plurality of the constituents thereof. The reactive gas is usually preheated by being passed through a preheater zone situated in the reaction chamber and into which the reactive gas inlet opens out. That method corresponds to the free-flow chemical vapor infiltration method.
In an industrial installation for chemical vapor infiltration, it is common practice to load the reaction chamber with a plurality of substrates or preforms that are to be densified simultaneously so as to increase the yield of the densification method, and consequently increase the packing density with which reaction chambers are loaded. Nevertheless, using free-flow chemical vapor infiltration to densify a plurality of substrates in a common chamber leads to certain difficulties, in particular relating to the uniformity of the resulting densification. When densifying thin substrates, e.g. in the form of fine rectangular plates disposed longitudinally in a reaction chamber with a reactive gas being diffused in free flow from the top edge thereof, it has been found that densification gradients are present within the substrates and between substrates within a single chamber (dispersion), and that this arises in spite of the care with which infiltration conditions are controlled. These deposition gradients are due in particular to lack of control over the flow of reactive gas within the chamber (privileged flow paths appear), thus leading to premature depletion of reagents and consequently to dispersions in densification between those portions of the substrates that are closest to and those that are furthest from the gas admission points.
In addition to the observed lack of uniformity in deposition, the densification of thin substrates presently also requires the use of support tooling so as to limit the extent to which the parts deform as a result of the deposition gradient and/or of internal stresses in the material. The use of such tooling penalizes the density with which the chamber can be loaded.
Procedures and installations for densifying porous annular substrates by chemical vapor infiltration are described in particular in documents US 2004/237898 and U.S. Pat. No. 5,904,957. Nevertheless, those methods apply essentially to densifying substrates of annular shape disposed in stacks, and they are not adapted to densifying substrates presenting thin shapes.